Document server code structure

Document the server code structure: Schedules, Jobs, Transactions, BackendTransactions and backend specific implementations.

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Summary

This MR introduces documentation for the server's code structure.

Additionally, a lot of documentation files received smaller fixes, for example dead links were replaced.

Focus

• Is the documentation content correct?
• Can the documentation be understood by new developers?
• Are the documentation pages (html and images) produced correctly?

Test Environment

• make doc, then direct your browser to build/doc/html/development/structure.html
• Test artefact generates by the pipeline (deploy > pages_prepare).

Check List for the Author

Please, prepare your MR for a review. Be sure to write a summary and a focus and create gitlab comments for the reviewer. They should guide the reviewer through the changes, explain your changes and also point out open questions. For further good practices have a look at our review guidelines

• All automated tests pass
• Reference related Issues
• Up-to-date CHANGELOG.md
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
• The test environment setup works and the intended behaviour is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?
• Removed f-doc-structure line from .gitlab-ci.yml before merging.

For further good practices have a look at our review guidelines.